TH350 Transmission Rebuild & Repair in Glendale, AZ (GM)

Sun West Transmissions — Glendale, AZ. Free estimates, honest diagnosis and a 3-year / 40,000-mile warranty on qualifying rebuilds and replacements.

The Classic Turbo 350

The Turbo-Hydramatic 350, usually called the TH350 or Turbo 350, is one of the most popular automatic transmissions in American automotive history. GM introduced this three-speed automatic around the end of the 1960s, and it was used through the 1970s and into the 1980s in a huge range of Chevrolet, Pontiac, Oldsmobile, Buick and GMC cars and light trucks.

Today the TH350 lives on in classic cars, restored pickups, street rods and project vehicles. Arizona’s dry climate preserves old vehicles well, so the Valley is full of classic Chevys, Camaros, Novas, C10 pickups and Chevelles running this transmission. Sun West Transmissions in Glendale rebuilds the TH350 for drivers who want a dependable stock unit or a stronger build for added power.

How the TH350 Works

The TH350 is fully hydraulic and mechanical, with no computer control. Shifts are controlled by a governor that responds to vehicle speed, a vacuum modulator that responds to engine load, and a kickdown cable or linkage that triggers downshifts at wide-open throttle. Inside, it uses several clutch packs, an intermediate overrun band, and a sprag and roller clutch for smooth engagement.

That simplicity is a big part of its appeal. There are no solenoids or software to troubleshoot. But because so many units are now decades old, wear, dried-out seals and previous repairs of unknown quality are common.

Common TH350 Problems

Leaks From Aged Seals

Front pump seals, rear seals, pan gaskets, the modulator and the shifter shaft seal can all leak after years of service or long periods of sitting. Desert heat dries out rubber and makes this worse, especially on project cars that sit in a hot garage for months at a time.

Vacuum Modulator Failure

A ruptured modulator diaphragm can let transmission fluid get pulled into the engine through the vacuum line. Signs include white exhaust smoke, low fluid and harsh or erratic shifts.

Slipping or Late Shifts

Worn clutches, a sticking governor, misadjusted kickdown linkage or vacuum leaks can cause slipping, late shifts or no upshift at all.

Loss of Second Gear

The intermediate sprag and intermediate clutch are known wear points. When they fail, the transmission may skip or slip in second gear.

Worn Bushings

Worn bushings allow pressure loss and can cause leaks, noise and delayed engagement.

Stock Rebuild or Performance Build?

Before we start, we’ll ask how you use the vehicle. A cruiser that goes to car shows and weekend drives has different needs than a hot rod with a built small-block that sees hard launches.

• Stock-style rebuild: Restores smooth, factory-like shifting and dependable operation for daily drivers and restorations.
• Heavy-duty rebuild: Adds upgraded clutches, a stronger intermediate sprag and improved components for more power or towing.
• Performance build: Includes firmer shift calibration, upgraded parts and a matched torque converter for modified engines.

See our transmission upgrades and performance transmission builds pages for more options.

Our TH350 Rebuild Process

1. Inspect the vehicle, check fluid, linkage, modulator and vacuum supply.
2. Remove and fully disassemble the transmission.
3. Clean and inspect all hard parts, including the pump, drums, gear sets and case.
4. Install new clutches, steels, seals, gaskets, bushings and filter.
5. Rebuild or replace sprags and roller clutches as needed.
6. Inspect the governor and valve body, and replace the modulator.
7. Evaluate the torque converter and install a new or matched unit if needed.
8. Reinstall, adjust kickdown and linkage, and road test.

TH350 Rebuild Cost

The cost of a TH350 rebuild depends on the condition of the core, how many hard parts need replacing, whether you want a stock, heavy-duty or performance build, and the type of torque converter. Vehicles with modified installations, such as custom crossmembers or aftermarket shifters, can also affect labor. Call us for a free quote based on your build.

Qualifying rebuilds include our 3-year / 40,000-mile warranty with no deductible on covered repairs. Read our warranty page for details and ask about financing.

We also work on the TH350’s bigger brother, the TH400, and the overdrive 700R4, a popular swap for better highway cruising.

TH350 Transmission FAQ

Should I swap my TH350 for an overdrive transmission?

If you drive on the freeway often, an overdrive unit like the 700R4 or 4L60E can lower cruising RPM. A rebuilt TH350 is a great choice for originality and simplicity.

Why is my classic car smoking white?

A failed vacuum modulator is one possible cause, but there are others. Have it checked to confirm.

How long does a TH350 rebuild last?

With proper installation, cooling and fluid service, a rebuilt TH350 can provide many years of dependable use.